click me

Is correct, although ‘&’ is more commonly used than ‘&’ or ‘,’.

If the ‘stringByAddingPercentEscapesUsingEncoding’ method does what it says on the tin, it should work(*), but the NSString documentation looks a bit unclear on which characters exactly are escaped. Check what you are ending up with, the URL should be something like:

mailto:bob@example.com?subject=test&body=Link%3A%3Ca%20href%3D%22http%3A//example.com/script%3Fp1%3Da%26amp%3Bp2%3Db%22%3Elink%3C/a%3E

(*: modulo the usual disclaimer that mailto: link parameters like ‘subject’ and ‘body’ are non-standard, will fail in many situations, and should generally be avoided.)

Once the email app opens, text stops at "...link:".

If ‘stringByAddingPercentEscapesUsingEncoding’ is not escaping ‘

In any case, you shouldn't expect the mailer to recognise the HTML and try to send an HTML mail; very few will do that.